Semiconductor device, electronic device, and semiconductor device manufacturing method

ABSTRACT

A semiconductor device, includes: a connection member including a first pad formed on a principal surface thereof; a semiconductor chip including a circuit-formed surface on witch a second pad is formed, the chip mounted on the connection member so that the circuit-formed surface faces the principal surface; and a solder bump that connects the first and second pads and is made of metal containing Bi and Sn, wherein the bump includes a first interface-layer formed adjacent to the second pad, a second interface-layer formed adjacent to the first pad, a first intermediate region formed adjacent to either one of the interface-layers, and a second intermediate region formed adjacent to the other one of the interface-layers and formed adjacent to the first intermediate region; Bi-concentration in the first intermediate region is higher than a Sn-concentration; and a Sn-concentration in the second intermediate region is higher than a Bi-concentration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2012-178509, filed on Aug. 10,2012, and the prior Japanese Patent Application No. 2011-221364, filedon Oct. 5, 2011, the entire contents of which are incorporated herein byreference.

FIELD

The embodiments discussed herein are related to a semiconductor device,an electronic device, and a method for manufacturing the same.

BACKGROUND

With an increase in integration density of semiconductor elements and anincrease in packaging density of electronic components, the number ofinput/output terminals of the semiconductor elements and the electronicdevices using the same is increasing. For example, in a semiconductorelement to be flip-chip mounted, the pitch between connection terminalsis reduced and further the area of the connection terminals is alsoreduced.

In order to achieve high-speed operation, severe demands are imposed oncurrent semiconductor elements in which high-speed operation is desired.For example, in a current high-speed semiconductor element, such as alarge scale integrated circuit (LSI), so-called low-K materials, such asporous silica, are used as an interlayer insulation film in order toreduce the parasitic capacitance between wiring patterns. However, thelow-K materials have problems in that the materials generally have a lowdensity corresponding to a low dielectric constant, and therefore thematerials are mechanically vulnerable and are easily damaged due tothermal distortion during joining. For example, porous silica has anelastic modulus of 4 to 8 GPa, and the mechanical strength thereof islower than that of conventional interlayer insulation materials, such asa silicon oxide film.

Under such a situation, the high-speed semiconductor elements containingthe low-K materials are desired to reduce thermal distortion of asubstrate during joining by joining the connection terminals at a lowtemperature when manufacturing a semiconductor device by flip-chipmounting of a semiconductor chip. However, a generally-used lead-freesolder for joining the connection terminals is used at a temperature of217° C. or higher for joining, and is not suitable for joining at such alow temperature. Under such a situation, in mounting of the high-speedsemiconductor elements containing the low-K materials, an eutecticSn(tin)-Bi (bismuth) solder having a melting point of 139° C. or asolder in which a little amount of elements, such as Ag, Cu, and Sb, isadded to Sn—Bi for the purpose of improving the mechanicalcharacteristics, such as ductility, is used as a solder material capableof reducing thermal stress in many cases.

As described above, the eutectic Sn—Bi solder has a melting point of139° C. and may be mounted at a temperature lower by about 80° C. than,for example, an Sn—Ag—Cu solder (Melting point of 217° C.) which is aconventional lead-free solder.

However, there is a demand in an actual electronic device such that, inorder to secure the reliability of the electronic device, the electronicdevice is subjected to a temperature cycle test or a high temperatureexposure test at an environmental temperature of about 150° C.considering the actual environment. However, when such a test isperformed, the environmental temperature (150° C.) of the test exceedsthe melting point (139° C.) of the Sn—Bi solder, which may cause aproblem of re-melting of a junction portion or the like.

In a semiconductor device or an electronic device having a configurationin which a large number of circuit boards and semiconductor chips arestacked, a problem may arise such that a portion, which is previouslyjoined by reflowing solder bumps, melts in the reflow of solder bumps tobe performed later in the semiconductor device or the electronic device.

Examples of the above-described related art is disclosed in KenichiYASAKA, Yasuhisa OHTAKE, et al., “Microstructural Changes in Micro-joinsbetween Sn-58Bi Solders and Copper by Electro-migration” ICEP 2010Proceedings FA2-1, pp. 475-478, and OHTAKE et al., “Electro-migration inMicrojoints between Sn—Bi Solders and Cu”, 16th Symposium onMicrojoining and Assembly Technology in Electronics, Feb. 2-3, 2010,Yokohama, pp 157-160.

SUMMARY

According to an aspect of the embodiments, a semiconductor deviceincludes: a first connection member that includes a first connection padformed on a first principal surface of the first connection member; afast semiconductor chip that includes a circuit-formed surface on whicha semiconductor integrated circuit is formed and a second connection padformed on the circuit-formed surface, the fast semiconductor chipmounted on the first connection member in such a manner that thecircuit-formed surface faces the first principal surface; and a solderbump that connects the first connection pad to the second connection padand is made of metal containing Bi and Sn, wherein the solder bumpincludes a first interface layer formed adjacent to the secondconnection pad, a second interface layer formed adjacent to the firstconnection pad, a first intermediate region formed adjacent to eitherone of the first interface layer or the second interface layer, and asecond intermediate region formed adjacent to the other one of the firstinterface layer and the second interface layer and formed adjacent tothe first intermediate region; a concentration of Bi in the firstintermediate region is higher than a concentration of Sn in the firstintermediate region; and a concentration of Sn in the secondintermediate region is higher than a concentration of Bi in the secondintermediate region.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a plan view illustrating the configuration of a semiconductordevice according to a first embodiment;

FIG. 1B is a cross sectional view along the IB-IB line of FIG. 1A;

FIG. 1C is a plan view illustrating an example of a wiring patternformed in members of FIG. 1A;

FIG. 2A is a cross sectional view illustrating the structure of a solderbump for use in the first embodiment;

FIG. 2B is a cross sectional view illustrating the structure of a solderbump according to one modification of the first embodiment;

FIG. 3A is a view (No. 1) illustrating a formation process of the solderbump of FIG. 2A;

FIG. 3B is a view (No. 2) illustrating a formation process of the solderbump of FIG. 2A;

FIG. 3C is a view illustrating a formation process of the solder bump ofFIG. 2B;

FIG. 4 is a phase diagram of a Sn—Bi binary system;

FIG. 5A is a SEM image illustrating the initial state of a solder bump;

FIG. 5B is a SEM image illustrating the final state of a solder bumpabout Sample 1;

FIG. 5C is a SEM image illustrating the final state of a solder bumpabout Sample 2;

FIG. 6 is a cross sectional view illustrating another modification of asolder bump;

FIG. 7A is a view (No. 1) explaining a first portion of a manufacturingprocess of a semiconductor device according to a second embodiment;

FIG. 7B is a view (No. 2) explaining a first portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 7C is a view (No. 3) explaining a first portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 7D is a view (No. 4) explaining a first portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 8A is another view (No. 1) explaining a second portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 8B is another view (No. 2) explaining the second portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 8C is another view (No. 3) explaining the second portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 8D is another view (No. 4) explaining the second portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 8E is another view (No. 5) explaining the second portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 8F is another view (No. 6) explaining the second portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 9A is another view (No. 1) explaining a third portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 9B is another view (No. 2) explaining the third portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 9C is another view (No. 3) explaining the third portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 9D is another view (No. 4) explaining the third portion of themanufacturing process of the semiconductor device according to thesecond embodiment;

FIG. 10 is a cross sectional view illustrating the configuration of asemiconductor device according to a third embodiment;

FIG. 11 is a cross sectional view illustrating the configuration of asemiconductor device according to a fourth embodiment; and

FIG. 12 is a perspective view illustrating an electronic deviceaccording to a fifth embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1A is a plan view illustrating the configuration of a semiconductordevice 20 according to a first embodiment. FIG. 1B illustrates a crosssectional view along the IB-IB line of FIG. 1A.

Referring to FIG. 1A and FIG. 1B, the semiconductor device 20 has acircuit board 11 (fast connection member) and a semiconductor chip 21(fast semiconductor chip), and the semiconductor chip 21 is flip-chipmounted on a mounting surface 11A (first principal surface) of thecircuit board 11.

When described in more detail, the semiconductor chip 21 has acircuit-formed surface 21A on which a large scale integrated circuit(LSI) is formed. On the circuit formed surface 21A, a large number ofelectrode pads 21 a (second connection pads 21a) containing copper (Cu),for example, are formed in a matrix shape. In contrast thereto, on thecircuit board 11, electrode pads 11 a (fast connection pads 11 a)corresponding to the electrode pads 21 a and similarly containing copperare formed in a matrix shape, for example, on the mounting surface 11Afacing the circuit formed surface 21A of the semiconductor chip 21.

The semiconductor chip 21 is mounted on the circuit board 11 in such amanner that the circuit formed surface 21A faces the mounting surface11A of the circuit board 11. The electrode pads 21 a are electricallyand mechanically connected to the corresponding electrode pads 11 a bySn—Bi solder bumps 31A.

On the mounting surface 11A of the circuit board 11, a large number ofwiring patterns 11 b each containing copper, for example, are formed asillustrated as the plan view of FIG. 1C. Each wiring pattern 11 bextends from the electrode pad 11 a to the electrode pad 11 c providedcorresponding to the electrode pad 11 a on the mounting surface 11A. Inthe circuit board 11, through via plugs 11C schematically illustratedwith the thick dashed line are formed corresponding to the electrodepads 11 c. The through via plug 11C extends from the mounting surface11A to a facing back surface 11B (second principal surface) through thecircuit board 11. FIG. 1C is a plan view of the mounting surface 11A ofthe circuit board 11 excluding the semiconductor chip 21. In FIG. 1C,the semiconductor chip 21 is illustrated with the thin dashed linecorresponding to the excluded state. On the back surface 11B, electrodepads 11 d having a size larger than that of the electrode pads 11 a areformed corresponding to the through via plugs 11C in a substantiallymatrix shape, for example, with a larger pitch than the pitch of theelectrode pads 11 a. On each electrode pad 11 d, solder bumps 11D largerthan the electrode pads 11 a are formed. The electrode pad 11 d may alsobe formed with copper and the solder bump 11D may also be formed fromthe same Sn—Bi solder as that of the solder bump 31A.

In the semiconductor device 20 of such a configuration, the electrodepads 21 a of the semiconductor chip 21 flip-chip mounted on the circuitboard 11 are electrically connected to the solder bumps 11D through thesolder bumps 31A, the electrode pads 11 a on the mounting surface 11A ofthe circuit board 11, the wiring patterns 11 b and the electrode pads 11c on the mounting surface 11A, the through electrodes 11C, and thecorresponding electrode pads 11 d. The circuit board 11 may be providedwith other active elements or passive elements on the mounting surface11A, in the circuit board 11, or further on the back surface 11B.

FIG. 2A is a cross sectional view illustrating the configuration of thesolder bump 31A in detail.

Referring to FIG. 2A, in this embodiment, a first interface layer 31 acontaining a copper-tin (Cu—Sn) alloy is formed contacting the electrodepad 21 a containing copper, a second interface layer 31 b containing acopper-tin alloy is formed contacting the electrode pad 11 a similarlycontaining copper, and a first intermediate region 31 c containing Bi(bismuth) in a concentration of 85 wt % or more as the main ingredientsis formed contacting the first interface layer 31 a in a laminated statein the solder bump 31A. Further, a second intermediate region 31 d isformed between the first intermediate region 31 c and the secondinterface layer 31 b. The second intermediate region 31 d containing acopper-tin alloy containing Sn in a high concentration is formed by areaction of concentrated Sn (tin) into a neighborhood of the secondinterface layer 31 b in the solder bump and the copper in the secondinterface layer 31 b.

For example, when the solder bump 31A has a diameter of about 100 μm,the first intermediate region 31 c and the second intermediate region 31d have a thickness reaching 65 μm and 35 μm, respectively, for example.

FIG. 2B illustrates a modification of the embodiment of FIG. 2A. In themodification of FIG. 2B, the first intermediate region 31 c is formedcontacting the second interface layer 31 b and the second intermediateregion 31 d is formed contacting the first interface layer 31 a.

The fast and the second intermediate regions 31 c and 31 d illustratedas FIG. 2A and FIG. 2B are formed by joining the semiconductor chip 21at a reflow temperature of 139□C, for example, onto the circuit board 11using an eutectic Sn—Bi solder as the solder bump 31A as describedlater, and then applying a direct current to the solder bump 31A toinduce electromigration and have a feature of having a melting pointexceeding 215□C, for example, which is still higher than that of theoriginal Sn—Bi solder.

The fast and the second intermediate regions 31 c and 31 d illustratedas FIG. 2A and FIG. 2B are formed by joining the semiconductor chip 21,for example, at a reflow temperature of 139° C. onto the circuit board11 using an eutectic Sn—Bi solder as the solder bump 31A, andsubsequently applying a direct current to the solder bump 31A to induceelectromigration, consequently, the solder bump 31A has a feature ofhaving a melting point exceeding 215° C., for example, which is stillhigher than that of the original Sn—Bi solder, as described later.

Therefore, it is noted that although the solder bump 31A of FIG. 2A andFIG. 2B is formed at a low reflow temperature, the solder bump 31A doesnot re-melt even when the environmental temperature increases almost tothe reflow temperature later and the electrical and mechanicalconnection between the semiconductor chip 21 and the circuit board 11 isstably maintained.

Hereinafter, formation processes of the structure of FIG. 2A aredescribed with reference to FIG. 3A and FIG. 3B.

Referring to FIG. 3A, the semiconductor chip 21 is joined onto thecircuit board 11 by reflowing a Sn—Bi solder bump 31Aa having asubstantially eutectic composition at a temperature of 139° C. in anitrogen gas atmosphere in this embodiment. By the heat treatmentaccompanied with the reflow, the first interface layer 31 a is formedwith a copper-tin alloy at a junction portion with the electrode pad 21a and the second interface layer 31 b is similarly formed with acopper-tin alloy at a junction portion with the electrode pad 11 a inthe solder bump 31Aa. Hereinafter, the state of FIG. 3A is referred toas an “initial state”.

Next, as illustrated as FIG. 3B, in this embodiment, a direct current Iis applied to the solder bump 31Aa using the electrode pad 21 a as ananode and the electrode pad 31 b as a cathode. It is known that when thedirect current I is applied to the Sn—Bi solder, Bi concentrates to theanode side and Sn concentrates to the cathode side by electromigration(Microstructural Changes in Micro-joins between Sn-58Bi Solders andCopper by Electro-migration ICEP 2010 Proceedings FA2-1, pp. 475-478 andOtake, et al., 16th Symposium on “Microjoining and Assembly Technologyin Electronics, Feb. 2-3, 2010, Yokohama).

Then, in this embodiment, segregation is induced in the solder bump 31Aawhich is uniform at the beginning utilizing the electro migrationphenomenon to form the first intermediate region 31 c abundant in Bi andthe second intermediate region 31 d abundant in Sn.

FIG. 4 is a phase diagram of a Sn—Bi binary system.

Referring to FIG. 4, when the Sn—Bi solder has a substantially eutecticcomposition, the melting point is about 139° C. Accordingly, thestructure of FIG. 3A may be formed by joining at such a low temperaturewithout producing an excessive thermal stress in the Low-K materials andthe like used in the semiconductor chip 21.

Furthermore, by performing the electrification process of FIG. 3B, theconcentration of Bi becomes higher in the first intermediate region 31 cthan that of the initial composition substantially corresponding to theeutectic composition, and thus the melting temperature of the firstintermediate region 31 c becomes higher than the melting temperature inthe initial composition. Similarly, also in the second intermediateregion 31 d, the concentration of Sn becomes higher than that of theinitial configuration, and also the melting temperature of the secondintermediate region 31 d becomes higher than that of initialconfiguration. More specifically, a preferable feature is obtained suchthat the melting temperature becomes higher in the solder bump 31A inwhich segregation has occurred as described above than the meltingtemperature of the solder bump 31Aa during joining. Hereinafter, thestate of FIG. 3B is referred to as a “final state”.

FIG. 5A is a SEM (scanning electron microscope) image, corresponding tothe initial state of FIG. 3A, of the section along the VA-VA line of thesolder bump 31Aa before the application of the direct current Iimmediately after the reflow.

Referring to FIG. 5A, it is found that a characteristic organization isformed in an eutectic alloy where a bright domain abundant in Bi and adark domain abundant in Sn are almost uniformly mixed in the solder bump31Aa.

In contrast thereto, FIG. 5B illustrates the cross-sectional structurealong the VB-VB line of FIG. 3B of the solder bump 31A afterelectrification, i.e., in the final state.

Referring to FIG. 5B, an alloy (intermetallic compound) layer having acomposition of Cu₆Sn₅ is formed as the first interface layer 31 a alongthe surface of the electrode pad 21 a. Moreover, an alloy (intermetalliccompound) layer having a composition of Cu₃Sn is formed as the secondinterface layer 31 b along the surface of the electrode pad 11 a.

Furthermore, the first intermediate region 31 c mainly containing Bi andsubstantially not containing Sn is formed in the shape of a layeradjacent to the first interface layer 31 a, and a region mainlycontaining a Cu₆Sn₅ alloy (intermetallic compound) and substantially notcontaining Bi is formed in the shape of a layer as a whole between thefirst intermediate region 31 c and the second interface layer 31 b toform the second intermediate region 31 d. The organization of FIG. 5B isobtained when a direct current is applied in the structure of FIG. 5A ata current density of 1.0 to 2.0×10⁸ Am⁻² without heating from theelectrode pad 21 a to the electrode pad 11 a, this is corresponding toExample 1 described later.

FIG. 5C represents the cross-sectional structure in the final state ofanother sample corresponding to Example 2 described later along theVC-VC line of the solder bump 31A after electrification of FIG. 3B.

Referring to FIG. 5C, an alloy (intermetallic compound) layer having acomposition of Cu₆Sn₅ is formed as the first interface layer 31 a alongthe surface of the electrode pad 21 a in the same manner as in the caseof FIG. 5B and an alloy (intermetallic compound) layer having acomposition of Cu₃Sn is formed as the second interface layer 31 b alongthe surface of the electrode pad 11 a.

Furthermore, also in the organization of FIG. 5C, the first intermediateregion 31 c mainly containing Bi and substantially not containing Sn isformed adjacent to the interface layer 31 a in the shape of a layer.Furthermore, between the first intermediate region 31 c and the secondinterface layer 31 b, a region mainly containing a Cu₆Sn₅ alloy(intermetallic compound) and substantially not containing Sn is formedin the shape of a layer to form the second intermediate region 31 d. Theorganization of FIG. 5C is obtained when a direct current is applied inthe structure of FIG. 5A at a current density of 1.0 to 2.0×10⁸ Am⁻²while heating the junction portions of the electrode pad 21 a and theelectrode pad 11 a to be connected by the SnBi solder 31Aa to 100° C. orhigher from the electrode pad 21 a to the electrode pad 11 a.

The results described in FIGS. 5B and 5C illustrate that Cu moves bydiffusion into the solder bump 31Aa from the electrode pad 11 a actingas the cathode with the application of the direct current and the Cuwhich moves by diffusion forms the interface layer 31 b and theintermediate region 31 d in the solder bump 31A by a reaction with Snpresent in the solder bump 31Aa. The results of FIGS. 5B and 5Cillustrate that Cu moves by diffusion into the solder bump 31Aa from theelectrode pad 21 a acting as the anode with the application of thedirect current and the Cu which moves by diffusion forms the interfacelayer 31 a in the solder bump 31A by a reaction with Sn present in thesolder bump 31Aa.

Hereinafter, specific Examples are described.

Example 1

The electrode pad 21 a was formed on the circuit formed surface 21A ofthe semiconductor chip 21 with a film thickness of 10 μm by electrolyticplating of a Cu film. The electrode pad 11 a was formed on the mountingsurface 11A (fast principal surface) of the circuit board 11 also with afilm thickness of 10 μm by electrolytic plating of a Cu film. Then, thesemiconductor chip 21 was mounted on the circuit board 11 by reflowingthe solder bump 31A at a temperature of 139° C. in a nitrogen gasatmosphere corresponding to the process of FIG. 3A using a Sn—Bi soldercontaining a Bi composition in a proportion of 40 wt % to 70 wt % andhaving a substantially eutectic composition as the solder bump 31Aa.

Furthermore, using the electrode pad 21 a as an anode and the electrodepad 11 a as a cathode, the direct current I was applied to the solderbump 31A from the anode 21 a side to the cathode 11 a side in that stateover 5 hours, in other words, an electron flow e⁻ from the cathode 11 aside to the anode 21 a side was applied. In this experiment, the solderbump 31Aa was not intentionally heated from the outside during theelectrification.

One obtained by such an experiment is the solder bump 31A having thelayer organization illustrated above in FIG. 5B in which segregation ofBi and Sn occurred.

The semiconductor device 20 in which the semiconductor chip 21 wasflip-chip mounted on the circuit board 11 thus obtained was confirmedfor electrical connection, and then was subjected to 500 cycles of atemperature cycle test at temperatures between −5° C. and +125° C. Then,it was confirmed that an increase in resistance of the connectingportions was suppressed to 10% or lower by the solder bumps 31A.Moreover, the same semiconductor device 20 was allowed to stand in anenvironment where the temperature was 121° C. and the humidity was 85%for 1000 hours, and then the resistance of the connecting portions wasinvestigated. Then, it was confirmed that an increase in resistance was10% or lower.

Example 2

The electrode pad 21 a was formed on the circuit formed surface 21A ofthe semiconductor chip 21 with a film thickness of 10 μm by electrolyticplating of a Cu film. The electrode pad 11 a was formed on the mountingsurface 11A of the circuit board 11 also with a film thickness of 10 μmby electrolytic plating of a Cu film. A flux was applied to the surfaceof the electrode pads 21 a and 11 a, and then the semiconductor chip 21was mounted on the circuit board 11 by reflowing the solder bump 31A ata temperature of 139° C. in a nitrogen gas atmosphere corresponding tothe process of FIG. 3A using a Sn—Bi solder containing a Bi compositionin a composition of 40 wt % to 70 wt % and having a substantiallyeutectic composition as the solder bump 31Aa.

Furthermore, using the electrode pad 21 a as an anode and the electrodepad 11 a as a cathode, the direct current I was applied to the solderbump 31A from the anode 21 a side to the cathode 11 a side in that stateover 5 hours, in other words, an electron flow e⁻ was applied from thecathode 11 a side to the anode 21 a side. In this experiment, thetemperature of the solder bump 31Aa was increased to a temperature of100° C. or higher and 139° C. or lower, which was the original meltingpoint, by heating from the outside during the electrification.

One obtained by such an experiment is the solder bump 31A having thelayer organization illustrated above in FIG. 5C in which segregation ofBi and Sn occurred.

The semiconductor device 20 in which the semiconductor chip 21 wasflip-chip mounted on the circuit board 11 thus obtained was confirmedfor electrical connection, and then was subjected to 500 cycles of atemperature cycle test at temperatures between −25° C. and +125° C.Then, it was confirmed that an increase in resistance of the connectingportions was suppressed to 10% or lower by the solder bumps 31A.Moreover, the same semiconductor device 20 was allowed to stand in anenvironment where the temperature was 121° C. and the humidity was 85%for 1000 hours, and then the resistance of the connecting portions wasinvestigated. Then, it was confirmed that an increase in resistance was10% or lower.

Example 3

Thus, in this embodiment, copper (Cu) can be used as the electrode pads21 a and 11 a. However, in addition thereto, other metal elementsforming an intermetallic compound with Sn, such as, nickel (Ni), canalso be used.

The electrode pad 21 a was formed on the circuit formed surface 21A ofthe semiconductor chip 21 with a film thickness of 10 μm by electrolyticplating of a nickel (Ni) film. The electrode pad 11 a was formed on themounting surface 11A of the circuit board 11 also with a film thicknessof 10 μm by electrolytic plating of a nickel film. A flux was applied tothe surface of the electrode pads 21 a and 11 a, and then thesemiconductor chip 21 was mounted on the circuit board 11 reflowing thesolder bump 31A at a temperature of 139° C. in a nitrogen gas atmospherecorresponding to the process of FIG. 3A using a Sn—Bi solder containinga Bi composition in a proportion of 40 wt % to 70 wt % and having asubstantially eutectic composition as the solder bump 31Aa.

Furthermore, using the electrode pad 21 a as an anode and the electrodepad 11 a as a cathode, the direct current I was applied to the solderbump 31A from the anode 21 a side to the cathode 11 a side in that stateover 5 hours, in other words, an electron flow e⁻ was applied from thecathode 11 a side to the anode 21 a side.

The semiconductor device 20 in which the semiconductor chip 21 wasflip-chip mounted on the circuit board 11 thus obtained was confirmedfor electrical connection, and then was subjected to 500 cycles of atemperature cycle test at temperatures between −25° C. and +125° C.Then, it was confirmed that an increase in resistance of the connectingportions was suppressed to 10% or lower by the solder bumps 31A.Moreover, the same semiconductor device 20 was allowed to stand in anenvironment where the temperature was 121° C. and the humidity was 85%for 1000 hours, and then the resistance of the connecting portions wasinvestigated. Then, it was confirmed that an increase in resistance was10% or lower.

Example 4

As described above, in this embodiment, although not only copper butnickel may be used as the electrode pads 21 a and 11 a, other metalelements forming an intermetallic compound with Sn, e.g., antimony (Sb),palladium (Pd), silver (Ag), gold (Au), platinum (Pt), cobalt (Co), andthe like may be used.

The electrode pad 21 a was formed on the circuit formation surface 21Aof the semiconductor chip 21 with a film thickness of 3 to 4 μm byelectrolytic plating of a palladium (Pd) film.

The electrode pad 11 a was formed on the mounting surface 11A of thecircuit board 11 also with a film thickness of 3 to 4 μm by electrolyticplating of a palladium film. A flux was applied to the surface of theelectrode pads 21 a and 11 a, and then the semiconductor chip 21 wasmounted on the circuit board 11 by reflowing the solder bump 31A at atemperature of 139° C. in a nitrogen gas atmosphere corresponding to theprocess of FIG. 3A using a Sn—Bi solder containing a Bi composition in aproportion of 40 wt % to 70 wt % and having a substantially eutecticcomposition as the solder bump 31Aa.

In this embodiment, an Sn—Bi alloy layer having a Sn concentrationhigher than that of the initial composition may also be formed as thesecond intermediate region 31 d as illustrated as FIG. 6 by reducing thecurrent density during electrification or by reducing theelectrification time.

Furthermore, using the electrode pad 21 a as an anode and the electrodepad 11 a as a cathode, the direct current I was applied to the solderbump 31A from the anode 21 a side to the cathode 11 a side in thatstate, in other words, an electron flow e⁻ was applied from the cathode11 a side to the anode 21 a side over 3 hours.

The semiconductor device 20 in which the semiconductor chip 21 wasflip-chip mounted on the circuit board 11 thus obtained was confirmedfor electrical connection, and then was subjected to 500 cycles of atemperature cycle test at temperatures between −25° C. and +125° C.Then, it was confirmed that an increase in resistance of the connectingportion was suppressed to 10% or lower by the solder bump 31A. Moreover,the same semiconductor device 20 was allowed to stand in an environmentwhere the temperature was 121° C. and the humidity was 85% for 1000hours, and then the resistance of the connecting portion wasinvestigated. Then, it was confirmed that an increase in resistance was10% or lower.

In this embodiment, the direction of applying the direct current is notlimited to the direction from the electrode pad 21 a to the electrodepad 11 a as illustrated as FIG. 3B and may also be set to the directionfrom the electrode pad 11 a to the electrode pad 21 a illustrated asFIG. 3C. In this case, the electrode pad 11 a serves as an anode and theelectrode pad 21 a serves as cathode and, in the solder bump 31A, astructure was formed such that the first intermediate region 31 c wasformed adjacent to the second interface layer 31 b and the secondintermediate region 31 d was formed adjacent to the first interfacelayer 31 a as described above with reference to FIG. 2B.

Second Embodiment

Hereinafter, a method for manufacturing the semiconductor device 20according to a second embodiment is described with reference to FIG. 7Ato FIG. 7D, FIG. 8A to FIG. 8D, FIG. 9A, FIG. 9B, FIG. 10A, FIG. 10B,and FIG. 11A to FIG. 11D.

Referring to FIG. 7A, on the circuit formed surface 21A of thesemiconductor chip 21, a thin copper film or nickel film 21 s is formedby a sputtering method, for example, as a seed layer for electrolyticplating with a film thickness of 50 nm to 200 nm. Furthermore, asillustrated as FIG. 7B, a resist film R₁ having an opening portion R₁Acorresponding to the electrode pad 21 a to be formed is formed on theseed layer 21 s.

Then, by immersing the structure of FIG. 7B in an electrolytic platingbath of copper or nickel, and performing electrolytic plating using theseed layer 21 s as an electrode, the electrode pad 21 a containingcopper or nickel is formed on the seed layer 21 s corresponding to theopening portion R₁A with a film thickness of 1 μm to 5 μm, for example,as illustrated as FIG. 7C.

Then, as illustrated as FIG. 7D, by removing the resist film R₁, thestructure is obtained in which the electrode pad 21 a is formed on theseed layer 21 s covering the circuit formed surface 21A of thesemiconductor chip 21.

In contrast, a thin copper film or nickel film 11 s is formed with afilm thickness of 50 nm to 200 nm as a seed layer for electrolyticplating by a sputtering method, for example, on the mounting surface 11Aof the circuit board 11 as illustrated as FIG. 8A. Then, as illustratedas FIG. 8B, on the seed layer 11 s, a resist film R₂ having an openingportion R₂A corresponding to the electrode pad 11 a to be formed isformed.

By immersing the structure of FIG. 8B in an electrolytic plating bath ofcopper or nickel, and performing electrolytic plating using the seedlayer 11 s as an electrode, the electrode pad 11 a containing copper ornickel is formed on the seed layer 11 s corresponding to the openingportion R₂A with a film thickness of 1 μm to 5 μm, for example, asillustrated as FIG. 8C.

Then, as illustrated as FIG. 8D, by removing the resist film R₂, thestructure is obtained in which the electrode pad 11 a is formed on theseed layer 11 s covering the mounting surface 11A of the circuit board11.

Then, in this embodiment, as illustrated in FIG. 8E, a resist film R₃ isformed on the structure of FIG. 8D, and then the resist film R₃ isexposed and developed to form a resist pattern R₃A as illustrated asFIG. 8F, so that the resist pattern R₃A protects a portion correspondingto the wiring pattern 11 b formed on the mounting surface 11A of thecircuit board 11 previously described with reference to FIG. 1C.

Then, as illustrated as FIG. 9A, a solder bump 31Aa containing a Sn—Bialloy and having an initial composition closer to the eutecticcomposition, for example, is supported on the electrode pad 21 a in thestructure of FIG. 7D through a flux layer. Then, the semiconductor chip21 in which the solder bump 31Aa is supported on the electrode pad 21 aas described above is placed on the circuit board 11 in such a mannerthat the circuit formed surface 21A faces the mounting surface 11A ofthe circuit board 11, and the solder bump 31Aa is abutted on theelectrode pad 11 a on the mounting surface 11A.

Then, the solder bump 31Aa of the initial composition is reflowed at atemperature of 139° C. in the state, and the semiconductor chip 21 ismounted on the circuit board 11 through the solder bump 31Aa.

Next, as illustrated as FIG. 9B, a direct-current power supply 35 isconnected between the seed layer 21 s and the seed layer 11 s, and thenthe direct current I is applied to the solder bump 31Aa from theelectrode pad 21 a as an anode to the electrode pad 11 a as a cathode,in other words, an electron flow e⁻ is applied from the electrode pad 11a as a cathode to the electrode pad 21 a as an anode.

As a result, as previously described with reference to FIG. 3A and FIG.3B, Bi concentrates to the electrode pad 21 a, i.e., a side near theanode, in the solder bump 31Aa of the initial composition to form thefirst intermediate region 31 c, and Sn concentrates to the electrode pad11 a, i.e., a side near the cathode, to form the second intermediateregion 31 d, so that the solder bump 31Aa of the initial compositionchanges to the solder bump 31A.

When the direction of the direct current I is reversed in the process ofFIG. 9B, the structure previously described with reference to FIG. 2B isobtained in which the second intermediate region 31 d is formed adjacentto the first interface layer 31 a and the first intermediate region 31 cis formed adjacent to the second interface layer 31 b.

Next, as illustrated as FIG. 9C, the structure of FIG. 9B is immersed inan etchant 37, for example, containing potassium hydrogensulfate as themain ingredients, for example, for 1 minute. Thus, the seed layer 21 sand a portion of the seed layer 11 s which is not protected by theresist pattern R₃A are removed by etching. This etching is carried outonly for removing the thin seed layers 21 s and 11 s, and the thickelectrode pads 21 a and 11 a are not substantially affected.

After pulling up from the etchant 37, the resist pattern R₃A is removedby, for example, a peeling liquid or the like, ashing in oxygen plasma,or the like to complete the semiconductor device 20 of the configurationin which the semiconductor chip 21 is electrically and mechanicallyconnected onto the circuit board 11 through the solder bump 31A and thepredetermined wiring pattern 11 b is formed on the mounting surface 11Aof the circuit board 11.

In this embodiment, any one of the processes of FIG. 7A to FIG. 7D andany one of the processes of FIG. 8A to FIG. 8F may be performed first,and may be simultaneously performed at the same time.

Although the same wiring pattern is also formed on the back surface 11Bof the circuit board 11 in this embodiment, the explanation is omitted.

According to the above-described embodiments, by joining a semiconductorchip and a circuit board or a first connection member and a secondconnection member by reflowing solder bumps containing a Sn—Bi alloy,and then applying a direct current to the solder bumps, a region wherethe Bi concentration is high and a region where the Sn concentration ishigh may be formed in such a manner as to be isolated from each other inthe solder bumps. Therefore, the melting temperature of the solder bumpsmay be made higher than the initial melting temperature.

Third Embodiment

FIG. 10 is a cross sectional view illustrating the outline of asemiconductor device 40 according to a third embodiment.

Referring to FIG. 10, the semiconductor device 40 has a packagesubstrate 41 having principal surfaces 41A and 41B forming the frontsurface and the back surface, respectively. An interposer 42 mounted onthe principal surface 41A of the package substrate 41 by Sn—Bi solderbumps 41 a, and the interposer 42 is correspondent to the circuit board11 in the embodiments above. Additionally, a large number of thesemiconductor chips 21 mounted on the interposer 42 by solder bumparrays 431A each containing the solder bumps 31A, in which a largenumber of circuit patterns 42Ckt are formed by a multilayerinterconnection structure in the interposer 42. Further, on theprincipal surface 41B of the package substrate 41, different solderbumps 41 b for mounting on a system board or the like are formed.

Further, although not illustrated, circuits by a multilayerinterconnection structure are formed on the principal surfaces 41A and41B of the package substrate 41.

When assembling such a semiconductor device 40, in order to reducethermal stress to the semiconductor chips 21, the semiconductor chips 21are mounted on the interposer 42 using a Sn—Bi solder having a usualeutectic composition in the solder bump arrays 431A. Thereafter, whenthe interposer 42 is mounted on the package substrate 41, or,furthermore, when the package substrate 41 is mounted on a system boardor the like of an electronic device, later, a problem arises in that thesolder bumps constituting the solder bump arrays 431A re-malt with theheat treatment for reflowing the solder bumps 41 a and 41 b.

In order to solve the problem, in this embodiment, when thesemiconductor chip 21 is mounted on the interposer 42, a direct currentis applied to the solder bump 31A to isolate the bump 31A to a regionabundant in Bi, i.e., the first intermediate region 31 c, and a regionabundant in Sn, i.e., the second intermediate region 31 d, as describedin the embodiments above. Therefore, the melting temperature of theentire solder bump 31A increases from the initial temperature inmounting, e.g., 139° C., to 215° C. or higher. Therefore, even whenreflowing the solder bump 41 a or 41 b later, the solder bumps 31A doesnot re-melt.

Similarly, in this embodiment, also with respect to the solder bump 41a, the interposer 42 is mounted on the package substrate 41, and then adirect current is applied to thereby isolate each solder bump 41 a to aregion abundant in Bi and a region abundant in Sn therein. Therefore,the melting temperature of the solder bump 41 a becomes higher than thetemperature during reflow, so that a problem of re-melting or the likeof the solder bump 41 a does not occur when mounting the packagesubstrate 41. Also when the semiconductor device 40 is subjected to athermal cycle test and a high temperature exposure test, the connectiondoes not become poor.

Thus, according to this embodiment, in the configuration in which alarge number of components are stacked while mounting by solder bumps,the melting temperature of the solder bumps may be increased aftermounting, so that a high-reliable electronic device may be manufacturedat a high yield.

Fourth Embodiment

FIG. 11 is a cross sectional view illustrating the configuration of asemiconductor device 60 according to a fourth embodiment.

Referring to FIG. 11, the semiconductor device 60 has a circuit board 61having principal surfaces 61A and 61B, in which a semiconductor chip 62is joined onto the principal surface 61A of the circuit board 61 in aface-up state through a resin layer 62C, i.e., the circuit formedsurface on which a semiconductor integrated circuit is formed is theupper side, in other words facing a side opposite to the circuit board61.

Furthermore, the semiconductor chip 21 is mounted on the semiconductorchip 62 in a face-down state through the solder bump array 431Adescribed above, and the semiconductor chip 62 is electrically connectedto the circuit pattern formed on the principal surface 61A of thecircuit board 61 by bonding wires 62A and 62B.

On the principal surface 61A, the semiconductor chips 62 and 21 aresealed together with the bonding wires 62A and 62B with a sealing resin63 and a large number of through vias 61 t are formed in the circuitboard 61. The circuit pattern on the principal surface 61A iselectrically connected to the circuit pattern formed on the principalsurface 61B through the through vias 61 t.

On the principal surface 61B, a large number of solder bumps 61 b areformed. The circuit board 61 is mounted on a system board, for example,of various electronic devices, such as a server, through the solderbumps 61 b.

Also in this embodiment, the solder bumps 31A constituting the solderbump array 431A are electrified after reflowing at a low temperature of139° C. as described above, and, as a result, the melting temperatureincreases to 215° C. or higher, for example.

Therefore, even when the semiconductor device 60 is mounted on anothersubstrate by reflowing the solder bumps 61 b and even when theelectronic device thus formed is subjected to various thermal cycletests and high temperature exposure tests, the solder bumps 31Aconstituting the solder bump array 431A do not re-melt.

Thus, according to this embodiment, a high-reliable semiconductor devicemay be manufactured at a high yield.

Fifth Embodiment

The semiconductor devices according to various embodiments describedabove may be variously applied, e.g., from application to electronicdevices for so-called high-end use, such as a server 70 having a systemboard 71, as illustrated as FIG. 12 to application to a circuit wiringboard of electronic devices for popular use, such as cellular phones anddigital cameras.

Referring to FIG. 12, the semiconductor device 40 of FIG. 10 or thesemiconductor device 60 of FIG. 11, for example, is flip-chip mounted onthe system board 71 together with a memory module 71B and the likethrough the solder bumps 41 b or 61 b in a state where a heatdissipation member 71A is supported.

Preferable embodiments are described above but embodiments are notlimited to specific embodiments and may be variously modified andaltered within the scope of the claims.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A semiconductor device, comprising: a firstconnection member that includes a first connection pad formed on a firstprincipal surface of the first connection member; a fast semiconductorchip that includes a circuit-formed surface on which a semiconductorintegrated circuit is formed and a second connection pad formed on thecircuit-formed surface, the fast semiconductor chip mounted on the firstconnection member in such a manner that the circuit-formed surface facesthe first principal surface; and a solder bump that connects the firstconnection pad to the second connection pad and is made of metalcontaining Bi and Sn, wherein the solder bump includes a first interfacelayer formed adjacent to the second connection pad, a second interfacelayer formed adjacent to the first connection pad, a first intermediateregion formed adjacent to either one of the first interface layer or thesecond interface layer, and a second intermediate region formed adjacentto the other one of the first interface layer and the second interfacelayer and formed adjacent to the first intermediate region; aconcentration of Bi in the first intermediate region is higher than aconcentration of Sn in the first intermediate region; and aconcentration of Sn in the second intermediate region is higher than aconcentration of Bi in the second intermediate region.
 2. Thesemiconductor device according to claim 1, wherein the firstintermediate region substantially does not contain Sn and the secondintermediate region substantially does not contain Bi.
 3. Thesemiconductor device according to claim 1, wherein the firstintermediate region substantially does not contain Sn and the secondintermediate region is made of an alloy of Sn and Bi.
 4. Thesemiconductor device according to claim 1, wherein the secondintermediate region contains an intermetallic compound or a solidsolution of Sn and a metal element constituting either one of the firstor second connection pads which is adjacent to the other one of the fastand the second interface layers adjacent to the second intermediateregion.
 5. The semiconductor device according to claim 4, wherein themetal element is selected from a group consisting of copper, nickel,antimony, palladium, silver, gold, platinum, and cobalt.
 6. Thesemiconductor device according to claims 1, wherein the concentration ofBi in the first intermediate region exceeds 85 wt %.
 7. Thesemiconductor device according to claim 1, wherein the first connectionmember is a second semiconductor chip different from the fastsemiconductor chip.
 8. The semiconductor device according to claims 1,wherein the first connection member is an interposer that includes asecond principal surface facing the first major surface and includes athird connection pad formed on the second principal surface beingelectrically connected to the first connection pad.
 9. The semiconductordevice according to claim 8, further comprising: a wiring board, whereinthe semiconductor device being mounted on a principal surface of thewiring board using a second solder bump formed on the third connectionpad.
 10. An electronic device, comprising: a system board; and asemiconductor device that is flip-chip mounted on the system board,comprising: a first connection member that includes a first connectionpad formed on a first principal surface of the first connection member;a fast semiconductor chip that includes a circuit-formed surface onwhich a semiconductor integrated circuit is formed and a secondconnection pad formed on the circuit-formed surface, the fastsemiconductor chip mounted on the first connection member in such amanner that the circuit-formed surface faces the first principalsurface; and a solder bump that connects the first connection pad to thesecond connection pad and is made of metal containing Bi and Sn, whereinthe solder bump includes a first interface layer formed adjacent to thesecond connection pad, a second interface layer formed adjacent to thefirst connection pad, a first intermediate region formed adjacent toeither one of the first interface layer or the second interface layer,and a second intermediate region formed adjacent to the other one of thefirst interface layer and the second interface layer and formed adjacentto the first intermediate region; a concentration of Bi in the firstintermediate region is higher than a concentration of Sn in the firstintermediate region; and a concentration of Sn in the secondintermediate region is higher than a concentration of Bi in the secondintermediate region.
 11. A semiconductor device manufacturing method,comprising: forming a first connection pad on a first principal surfaceof a first connection member; forming a second connection pad on acircuit-formed surface of a fast semiconductor chip on which asemiconductor integrated circuit is formed; placing the fastsemiconductor chip on the first connection member in such a manner thatthe circuit-formed surface faces the first principal surface and thefirst connection pad contacts the second connection pad through a solderbump containing a Sn—Bi alloy; reflowing the solder bump for joining thefirst connection pad and the second connection pad; and applying adirect current after the joining between the first connection pad andthe second connection pads using either one of the first or secondconnection pad as an anode and using the other one of the first andsecond connection pads as cathode, so as to concentrate Bi in the solderbump into a neighborhood of the anode and to concentrate Sn in thesolder bump into a neighborhood of the cathode;
 12. The semiconductordevice manufacturing method according to claim 11, wherein the applyingthe direct current is performed while heating the solder bump.
 13. Thesemiconductor device manufacturing method according to claim 12, whereinthe applying the direct current is performed at a temperature of 100° C.or higher and the temperature at which the solder bump does not melt.14. The semiconductor device manufacturing method according to claim 11,wherein the applying the direct current is carried out at a currentdensity in a range of 1.0×10⁸ Am⁻² to 2.0×10⁸ Am⁻².
 15. Thesemiconductor device manufacturing method according to claim 11, whereinthe applying the direct current is performed by setting a currentdensity and an applying time in such a manner that a first intermediateregion substantially not containing Sn is formed at the neighborhood ofthe anode and a second intermediate region substantially not containingBi is formed at the neighborhood of the cathode.
 16. The semiconductordevice manufacturing method according to claim 11, wherein the applyingof the direct current is performed by setting a current density andapplying time in such a manner that a first intermediate regionsubstantially not containing Sn is formed at the neighborhood of theanode and a second intermediate region containing Sn and Bi is formed atthe neighborhood of the cathode.
 17. The semiconductor devicemanufacturing method according to claim 11, wherein the forming thefirst connection pad includes forming a first metal film that is formedon the first principal surface and is connected to the first connectionpad to serve as a first conductive path to the first connection pad; theforming the second connection pad includes forming a second metal filmthat is formed on the circuit formed surface and is connected to thesecond connection pad to serve as a second conductive path to the secondconnection pad; and the applying the direct current is carried outthrough the first conductive path and the second conductive path. 18.The semiconductor device manufacturing method according to claim 17,further comprising: after the applying the direct current, removing thefirst metal film and the second metal film by wet etching.
 19. Thesemiconductor device manufacturing method according to claims 11,wherein, in the process for applying the direct current, a material ofthe first and second connection pads moves by diffusion into the solderbump to form an intermetallic compound phase or a solid solution withtin.
 20. The semiconductor device according to claim 19, wherein thematerial is selected from copper, nickel, antimony, palladium, silver,gold, platinum, and cobalt.